Malaria drug hydroxychloroquine, which U.S. President Donald Trump says he has been taking, is tied to increased risk of death in COVID-19 patients, according to a study published in medical journal Lancet.

The study, which observed over 96,000 people hospitalized with COVID-19, showed that people treated with the drug, or the closely related drug chloroquine, had higher risk of death when compared to those who had not been given the medicine.

Demand for hydroxychloroquine, a drug approved decades ago, surged after Trump touted its use as a coronavirus treatment in early April. Earlier this week, he surprised the world by admitting he was taking the pill as a preventative medicine.

The Lancet study authors suggested these treatment regimens should not be used to treat COVID-19 outside of clinical trials until results from clinical trials are available to confirm the safety and efficacy of these medications for COVID-19 patients.

The authors said they could not confirm if taking the drug resulted in any benefit in coronavirus patients.

Weeks ago, Trump had promoted the drug as a potential treatment based on a positive report about its use against the virus, but subsequent studies found that it was not helpful. The U.S. Food and Drug Administration in April issued a warning about its use.

The Lancet study looked at data from 671 hospitals, where 14,888 patients were given either hydroxychloroquine or chloroquine, with or without the antibiotic macrolide, and 81,144 patients were not on any of the treatment regimens.